全文获取类型
收费全文 | 1820篇 |
免费 | 99篇 |
国内免费 | 8篇 |
专业分类
化学 | 1219篇 |
晶体学 | 34篇 |
力学 | 44篇 |
数学 | 102篇 |
物理学 | 310篇 |
无线电 | 218篇 |
出版年
2024年 | 16篇 |
2023年 | 21篇 |
2022年 | 84篇 |
2021年 | 101篇 |
2020年 | 64篇 |
2019年 | 75篇 |
2018年 | 97篇 |
2017年 | 76篇 |
2016年 | 97篇 |
2015年 | 68篇 |
2014年 | 75篇 |
2013年 | 150篇 |
2012年 | 150篇 |
2011年 | 130篇 |
2010年 | 84篇 |
2009年 | 71篇 |
2008年 | 84篇 |
2007年 | 67篇 |
2006年 | 46篇 |
2005年 | 38篇 |
2004年 | 28篇 |
2003年 | 30篇 |
2002年 | 51篇 |
2001年 | 34篇 |
2000年 | 39篇 |
1999年 | 11篇 |
1998年 | 17篇 |
1997年 | 10篇 |
1996年 | 5篇 |
1995年 | 8篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 11篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1974年 | 2篇 |
1937年 | 2篇 |
排序方式: 共有1927条查询结果,搜索用时 640 毫秒
981.
Nurul Asyikin Mohd Radzi Norashidah Md. Din Mohammed Hayder Al-Mansoori Sajaa Kh. Sadon Mohd Shahmi Abdul Majid 《Photonic Network Communications》2014,27(1):1-7
Ethernet Passive Optical Network (EPON) has emerged as a promising technology since it offers high bandwidth at a distance that can go up to 20 km. In the upstream EPON transmission, a dynamic bandwidth allocation (DBA) algorithm is needed to avoid collisions and to efficiently allocate the bandwidth. This paper proposes an algorithm called as a Universal DBA (UDBA) algorithm that can support the priority universally in the entire EPON system. A peripheral interface controller (PIC)-based EPON testbed is used to test the algorithm within the mulipoint control protocol environment. For the performance study, we have compared the results achieved using UDBA in terms of delay, throughput, and fairness with two other DBA algorithms. UDBA shows improvements in terms of expedited forwarding, assured forwarding and best effort delay as much as 43.9, 42.6, 20 % over User-oriented Hierarchical Bandwidth Scheduling with Single Cycle (UHSA-S) and 37.5, 20.2, 25.3 % over Modified Smallest Available Report First (MSARF), respectively. Throughput is improved as high as 17.9 % over UHSA-S and 12.8 % over MSARF. In addition, the fairness index of UDBA is higher up to 14 % as compared to UHSA-S and 2.7 % as compared to MSARF. 相似文献
982.
Abdul‐Majid Wazwaz 《Mathematical Methods in the Applied Sciences》2019,42(5):1553-1560
In this work, we employ the recursion operator, the Burgers equation and its inverse operator, for constructing a hierarchy of negative‐order integrable Burgers equations of higher orders. The complete integrability of each established equation emerges by virtue of the correlation between integrability and recursion operators. We use the simplified Hirota's method to obtain multiple kink solutions for some of the derived equations, and in particular, for the generalized negative‐order Burgers equation. 相似文献
983.
This paper reports a wind-powered water pumping system implemented in rural side of Pakistan. The design methodology presented in this paper shall enhance the efficiency of the existing Savonius turbine's performance by modifying it aerodynamically. Blades with different twist angles are designed in Solid Edge and analyzed using computational fluid dynamics (CFD) with ANSYS FLUENT software. Static and rotational analyses are performed to get optimized twist angle and results are highlighted. The performances of the turbine in both static and rotational analyses are compared. 相似文献
984.
Tractor beam for fully immersed multiple objects: Long distance pulling,trapping, and rotation with a single optical set‐up 下载免费PDF全文
Md. Masudur Rahman Ayed Al Sayem M. R. C. Mahdy Md. Ehsanul Haque Rakibul Islam S. Tanvir‐ur‐Rahman Chowdhury Md. Abdul Matin 《Annalen der Physik》2015,527(11-12):777-793
In this article, we have theoretically demonstrated the mechanism of an active tractor beam for multiple fully immersed objects with additional abilities to yielding stable long distance levitation, a controlled rotation and a desired 3D trapping. This is demonstrated with a single optical set‐up by using two coaxial, or even non‐coaxial, superimposed higher order monochromatic Bessel beams of reverse helical nature and different frequencies. The superimposed beams can possess periodic intensity variations both along and around the beam‐axis due to a difference in longitudinal wave‐numbers and beam orders, respectively. The difference in frequencies of the two laser beams makes the intensity pattern to move along and around the beam‐axis in a continuous way without manual ramping of phase, which allows for bidirectional movement of completely immersed multiple particles. The condition for increasing or decreasing the dimension of binding regions is also proposed here to manipulate multiple immersed objects of different sizes under dipole approximation.
985.
Yusuke Hirasawa Hiroko Arai Abdul Rahman Idha Kusumawati Noor Cholies Zaini Osamu Shirota Hiroshi Morita 《Tetrahedron》2013
Five new indole alkaloids, voacalgines A–E (1–5) consisting of a C-mavacurine type of skeleton with 2,3-dihydroxybenzoate moiety, a macroline-type of skeleton, or a macroline-type of skeleton with C6 unit, were isolated from the bark of Voacanga grandifolia. Their relative structures were determined by means of NMR data. Voacalgine A showed moderate cell growth inhibitory activities against HL-60 and HCT116 cells. 相似文献
986.
Sadia Khan Itrat Fatima Mehdi Hussain Kazmi Abdul Malik 《Helvetica chimica acta》2013,96(6):1176-1181
Allumines A and B ( 1 and 2 , resp.), two new steroidal alkaloids, and a new cyclopentene derivative, 3 , were isolated from the CHCl3‐soluble fraction of the whole plant of Allium victorialis. Their structures were elucidated by spectroscopic techniques, including 1D‐ and 2D‐NMR spectroscopy. 相似文献
987.
A concise and simple synthesis of M1 allosteric agonist 1-(1′-(2-methylbenzyl)-1,4′-bipiperidin-4-yl)-1H-benzo[d]imidazol-2(3H)-one (TBPB) using economical and commercially available orthophenylenediamine (O-PDA) and Boc piperidone has been described. The disclosed scheme allows synthesizing more analogs that were otherwise difficult to prepare with the original method. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file. 相似文献
988.
An efficient three-step methodology developed for the preparation of 1,2-amino alcohols. In the first step a rapid coupling between bromoketones and potassium phthalimide in ionic liquid produced α-phthalimido ketones in quantitative yields, which is followed by a facile reduction using NaCNBH3 in acetic acid to give corresponding phthalimido alcohols and finally effecting hydrazinolysis in water at 60 °C to yield biologically important 1,2-amino alcohols. 相似文献
989.
990.
Saloni Gangal Shweta Sharma Abdul Rauf 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):242-259
A microwave-assisted combinatorial synthesis of 2-long alkenyl chain benzoxazoles and naphtho[2,3-d]oxazoles with a catalytic amount of phosphorus pentasulphide at ambient pressure has been developed. This procedure constitutes a simple, practical, and green synthetic method for benzoxazoles and their structural analogs. All the compounds [2(a–d) through 6(a–d)] have been screened for antibacterial and antifungal activity. The compounds have showed good activity against Gram-positive and Gram-negative bacteria. All the compounds have also showed good results against almost all fungal strains. The structures of the synthesized compounds are elucidated by IR, 1H NMR, 13C NMR, MS data, and elemental analysis. 相似文献